Tainting means to spoil something by adding something bad or unpleasant to it.
Tainting has a negative feeling. It means making something worse or less good.
Examples:
The bad smell is tainting the air.
The rotten apple is tainting the other fruit.
The dirty water is tainting the clean water.
The loud music is tainting the peaceful park.
The lie is tainting the truth.
The spoiled milk is tainting the whole fridge.